The present invention relates to a control rod and fuel supporting member gripping apparatus for gripping a control rod (hereinafter referred to as CR) and a fuel supporting member ( hereinafter referred to as FS) in order to remove and carry the CR and FS out of the reactor and to load the CR and FS again in the reactor, and also relates to a method of withdrawing the control rod and fuel supporting member gripping apparatus.
Generally, the core of the boiling water reactor (BWR) is constructed as shown in FIG. 13 and a plurality of fuel assemblies 3 and CRs (control rods) 4 are mounted in a cylindrical core shroud 2 contained in a reactor pressure vessel 1.
The top portions of these fuel assemblies 3 are supported by means of an upper lattice plate 5 and the bottom portions thereof are supported by means of a core supporting plate 7 through FSs (fuel supporting member) 6.
The respective FSs 6 are supported by means of the core supporting plate 7 by engaging the cylindrical bottom portion thereof with an FS supporting engagement hole 7a as shown in FIG. 15. As shown in FIG. 16, the square top portion of the core supporting plate 7 has supporting engagement holes 6a, 6b, 6c, 6d for allowing the bottom of the fuel assembly 3 to engage therewith for supporting the fuel assembly 3 and has a cross shaped insertion hole for allowing the CR 4 to go through.
The respective supporting engagement holes 6a-6d communicate with respective orifices 6e, 6f, 6g, 6h which are located on the sides of the supporting engagement holes in order to allow coolant to flow into the respective fuel assemblies 3 from the respective orifices 6e-6h through the respective supporting through holes 6a-6d. A through hole 6i which engages with a fixing pin 7b implanted on the core supporting plate 7 is provided on a square corner of the top portion of the FS 6 in order to fix the FS 6 onto the core supporting plate 7.
On the other hand, the CR 4 is detachably connected to a control rod driving mechanism (hereinafter referred to as CRD) which is provided so as to vertically go through the bottom of the reactor pressure vessel 1 and the CR 4 is lifted up and down by means of each CRD 8 so as to be inserted into and pulled from the core.
The CR 4 passes through a CR guide pipe 9 which is connected to the core supporting plate 7 so that the CR 4 is lifted up and down through a cross shaped insertion hole 4a formed among the four bodies of the fuel assemblies 3, 3, 3, 3 which are supported by means of the FS 6.
A conventional coupling mechanism for the CR 4 and the CRD 8 is constructed in the form of a spud as shown in FIG. 17. As for the spud type coupling 10, a coupling spud 11 having locking pawls which are formed by incising the circumferential portion thereof so as to obtain, for example, four split parts is pushed up strongly by means of a driving piston, not shown, of the CRD 8 and then inserted into a gap around a lock plug 12 which is inserted into the engagement hole in the bottom portion 4b of the CR 4. Consequently, the coupling spud 11 is nipped between the internal face of the bottom portion 4b of the CR 4 and the external face of the lock plug 12 in order to connect the CR 4 with the CRD 8.
By pushing up the lock plug 12 by means of an uncoupling rod 13 of the CRD 8 to resist the force of a spring 4c, the CR 4 is disconnected from and released from the CRD 8.
When the CR 4 is removed from the CR driving mechanism in the reactor pressure vessel 1 and carried out of the core at the time of the periodic inspection of the BWR, first of all, the fuel assemblies 3 are pulled out of the core.
However, since in the conventional BWR, the connection between the CR 4 and the CRD 8 is released by pushing the lock plug 12 strongly by means of the uncoupling rod 13, if such a foreign matter as clad or the like is caught between the lock plug 12 and the coupling stud 11, the lock plug 12 sticks firmly to the coupling spud 11, so that it may be impossible to disconnect the CR 4 from the CRD 8.
To solve such a problem, recently a bayonet coupling 14 as shown in FIGS. 18A-18C has been sometimes employed as a connecting means for the CR 4 and the CRD 8.
The bayonet coupling 14 has engaging protrusions 16 having a specified width which are disposed at every 90.degree. along the circumference thereof, the engaging protrusions protruding out of the internal face of an engaging hole 15 in the bottom portion 4b of the CR 4 in which the coupling spud 11 is to be inserted. By turning the bayonet coupling 14 or the CR 4 by 45.degree. along the circumference thereof as shown in FIG. 18B, the respective engaging protrusions 16 are moved along the external face of the respective coupling spuds 11 to reduce the diameter of the respective coupling spuds 11. Consequently, the lock plug 17 is nipped to connect the CR 4 with the CRD 8.
If the CR 4 is turned further by 45.degree. or returned to its original position as shown in FIG. 18C, the respective coupling spuds 11 are moved to respective cavities of the engaging hole 15 to expand the diameter of the coupling spuds 11, thereby releasing connection between the CR 4 and the CRD 8.
In the CR 4 which employs the aforementioned bayonet coupling 14, no foreign matter such as clad or the like is caught between the lock plug 12 and the coupling spud 11 unlike the conventional spud type coupling 10. Thus, it is possible to release the CR 4 from the CRD 8 securely.
It is necessary to turn the bayonet coupling 14 or the CR 4 axially by 45.degree. to release the CR 4 from the CRD 8. However, because the CR 4 is inserted through the cross shaped insertion hole 4a of the FS 6 which is fixed by the fixing pin 7b of the core supporting plate 7, it is not possible to turn the CR 4. If the CR 4 is turned forcibly, the CR 4 and the FS 6 may be damaged.
If the CR 4 and the FS 6 are turned at the same time after the FS 6 is removed from the fixing pin 7b of the core supporting plate 7 so that the FS 6 is free, the top portion of the FS 6 collides with fuel assemblies in the lattice in the vicinity because the top portion thereof is square shaped, so that the fuel assemblies 3 may be damaged.
FIG. 19A shows a plan view of the CR 4 and the FS 6 viewed from the upper side of the upper lattice plate 5 in a steady state. As shown in FIG. 19A, the CR 4 is located in the same direction as that of the upper lattice plate 5, and the FS 6 has a shape capable of passing the cell 5' formed to the upper lattice plate 5. The FS 6 is provided with projections 6' which can nip a pin 7b provided to the core supporting plate 7 to thereby prevent the FS 6 from rotating. The fuel assemblies, each having a square cross section, are positioned on the fuel assembly supporting through holes 6a, 6b, 6c, 6d formed to the FS 6. A reactor core is constituted by about 100 units of fuel assemblies, each unit including the thus arranged four fuel assemblies.
When the CR 4 and the FS 6 now in the state of FIG. 19A are simultaneously gripped by the respective gripping devices and are then lifted to rotate them, the projections 6' of the FS 6 contact the surrounding fuel assemblies. This state is shown in FIG. 19B showing an arrangement in which the CR 4 and the FS 6 are rotated by 45.degree. from the arrangement shown in FIG. 19A. In this arrangement, the projections 6' project out of the cell 5', which may contact the fuel assembly disposed in the upper side cell and hence damage the same. In order to obviate such defect, as shown in FIG. 19C, sixteen fuel assemblies of the other four cells 5" surrounding the cell 5' now treated as well as the four fuel assemblies of the cell now treated have to be withdrawn upward from the core and conveyed to the fuel storage pool formed upper outside of the reactor pressure vessel, thus being troublesome and inconvenient in the prior art technology.